This invention relates to a process for inhibiting deposition of radioactive substances on nuclear power plant components such as primary cooling water piping contacting with cooling water containing radioactive substances.
Piping, pumps, valves and the like (hereinafter referred to as "components") used in a primary cooling water system in a nuclear power plant are made of stainless steel, Stellite, etc. When these metals are used for a long period of time, they are corroded and damaged to release constituting metal elements into a nuclear reactor cooling water (hereinafter referred to as "cooling water"), which is sent to the interior of nuclear reactor. The released metal elements change into almost oxides, which deposit on fuel sticks and are exposed to neutron irradiation. As a result, there are produced radionuclides such as .sup.60 Co, .sup.58 Co, .sup.51 Cr, .sup.54 Mn, etc. These radionuclides are released in the primary cooling water again to become ions or to float as insoluble solids (herein after referred to as "crud") therein. A part of ions or crud is removed by a demineralizer for cleaning a reactor water, but the remainder deposits on surfaces of the components while circulating in the primary cooling water system. Thus, the dose rate at the surfaces of components increases, which results in causing a problem of exposure to irradiation of workers at the time of inspection or for maintenance.
There have been proposed various processes for inhibiting the release of these metal elements which is a source of such a problem in order to lower the deposition of radiactive substances. For example, materials having good corrosion resistance are used, or oxygen is introduced into a water supply system in order to inhibit the corrosion of the components. But the corrosion of components of the water supply system and primary cooling water system cannot be inhibited sufficiently and the amount of radioactive substances in the primary cooling water cannot be reduced sufficiently, even if any processes are used. Therefore, the increase of dose rate at the surfaces of components due to the deposition of radioactive substances still remains as a problem.
On the other hand, various methods for removing deposited radioactive substances on the components have been studied and practically used. These methods can be divided into (1) mechanical cleaning, (2) electrolytic cleaning and (3) chemical cleaning. The methods of (1) and (2) are difficult to remove radioactive substances adhered to the component surfaces strongly, and cannot be used for systematic decontamination in a broad range. Therefore, the method (3) is widely used today. According to the method (3), a reagent solution such as an acid solution is used to dissolved an oxide film on steel surface by chemical reaction and to remove radioactive substances present in the oxide film. But there is a problem in the method (3) in that even if the dose rate may be reduced temporally, the components are rapidly contaminated again when exposed to a solution dissolving radioactive substances in high concentration.
In order to remove such a problem, there is proposed a process for inhibiting the deposition of radioactive substances by forming an oxide film on component surfaces previously (e.g. Japanese Patent Application Nos. 28976/79 and 146111/82). But according to this process, deposition behavior of radioactive substances changes remarkably depending on properties of oxide films previously formed. For example, behavior of radioactive ions is different depending on charged state of an oxide film previously formed, and the growth rate of oxide film newly formed on component surfaces after immersion in a solution for dissolving radioactive substances changes depending on properties of oxide film originally formed. Therefore, it is necessary to conduct an oxidation treatment of the components by a process best suited for applying solution.